A previous dual-circuit brake valve device with relay valve device is shown and disclosed in published German Application DT No. 25 48 973C2 and, more particularly, reference is made to FIG. 3 thereof. The second portion is designed as a relay valve to which is passed a control pressure to control the brake pressure, namely, first brake pressure that is supplied to the first brake line circuit.
In the following description, it will be understood that the term "control pressure" includes the "first brake pressure", while the term "consumer line or circuit" includes the "second brake line or circuit", and that the term "air pressure control system" includes the entire "motor vehicle brake control system" under the respective associated circumstances.
Presently, the state of the art relay valve has a relay piston having first and second control surfaces, in which only the first control surface is capable of receiving control pressure.
When pressure is applied to the first control surface, a second control pressure is essentially passed to the second control surface at the same time. In one case, the amount of pressure supplied from the relay valve to the consumer line or circuit depends upon the ratio between the first control surface and the reaction surface. On the other hand, it depends upon the ratio of the sum of the pressure appearing on the first and second control surfaces relative to the reaction surface, and also upon the pressure level of the first control pressure and the second control pressure. In the present conventional relay valves, the first control surface is smaller than the reaction surface, and the sum of the area of both control surfaces is essentially equal to the reaction surface, while the second control pressure (within a given range) is variable in relation to the control pressure and is equal to this, at the most. Due to this relay configuration of the present state of the art, the pressure supplied to the consumer line or circuit does not follow one or more definite given characteristic curves relative to the control pressure. Rather, the state of the art relay valve has a characteristic range in which the pressure supplied to the consumer line or circuit is reduced relative to the control pressure or is equal to the control pressure, most of the time.
There are some applications where a signal pressure is at an essentially constant level in the pressure control system and is independent from the control pressure and where the pressure which is supplied by a relay valve device into a consumer line or circuit, namely, the second brake pressure supplied into a second brake circuit, is intended to follow a first characteristic curve in accordance with the control pressure for as long as a signal pressure is present, and is intended to follow a second characteristic curve which is at a lower value when the signal pressure is no longer present. For this type of operation, the initially-described conventional relay valve device cannot be used because the appearance of the signal pressure to the second control surface would result in a continuous pressure in the consumer line or in the second brake circuit for the duration of the application.